Commit 820962dc authored Dec 23, 2015 by Rabin Vincent Committed by Steve French Jan 14, 2016

cifs: fix race between call_async() and reconnect()



cifs_call_async() queues the MID to the pending list and calls
smb_send_rqst().  If smb_send_rqst() performs a partial send, it sets
the tcpStatus to CifsNeedReconnect and returns an error code to
cifs_call_async().  In this case, cifs_call_async() removes the MID
from the list and returns to the caller.

However, cifs_call_async() releases the server mutex _before_ removing
the MID.  This means that a cifs_reconnect() can race with this function
and manage to remove the MID from the list and delete the entry before
cifs_call_async() calls cifs_delete_mid().  This leads to various
crashes due to the use after free in cifs_delete_mid().

Task1				Task2

cifs_call_async():
 - rc = -EAGAIN
 - mutex_unlock(srv_mutex)

				cifs_reconnect():
				 - mutex_lock(srv_mutex)
				 - mutex_unlock(srv_mutex)
				 - list_delete(mid)
				 - mid->callback()
				 	cifs_writev_callback():
				 		- mutex_lock(srv_mutex)
						- delete(mid)
				 		- mutex_unlock(srv_mutex)

 - cifs_delete_mid(mid) <---- use after free

Fix this by removing the MID in cifs_call_async() before releasing the
srv_mutex.  Also hold the srv_mutex in cifs_reconnect() until the MIDs
are moved out of the pending list.

Signed-off-by: Rabin Vincent <rabin.vincent@axis.com>
Acked-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <sfrench@localhost.localdomain>

parent 373512ec

fs/cifs/connect.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -370,7 +370,6 @@ cifs_reconnect(struct TCP_Server_Info *server)
		server->session_key.response = NULL;
		server->session_key.len = 0;
		server->lstrp = jiffies;
		mutex_unlock(&server->srv_mutex);

		/* mark submitted MIDs for retry and issue callback */
		INIT_LIST_HEAD(&retry_list);
		@@ -383,6 +382,7 @@ cifs_reconnect(struct TCP_Server_Info *server)
		list_move(&mid_entry->qhead, &retry_list);
		}
		spin_unlock(&GlobalMid_Lock);
		mutex_unlock(&server->srv_mutex);

		cifs_dbg(FYI, "%s: issuing mid callbacks\n", __func__);
		list_for_each_safe(tmp, tmp2, &retry_list) {

fs/cifs/transport.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -576,14 +576,16 @@ cifs_call_async(struct TCP_Server_Info server, struct smb_rqst rqst,
		cifs_in_send_dec(server);
		cifs_save_when_sent(mid);

		if (rc < 0)
		if (rc < 0) {
		server->sequence_number -= 2;
		cifs_delete_mid(mid);
		}

		mutex_unlock(&server->srv_mutex);

		if (rc == 0)
		return 0;

		cifs_delete_mid(mid);
		add_credits_and_wake_if(server, credits, optype);
		return rc;
		}

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